1. Field of the Invention
The present invention relates to an apparatus for hydraulically operating a clutch which is assembled or is contained in a manual transmission for a vehicle such as a motor vehicle, or the like.
2. Description of Related Art
This kind of conventional apparatus for hydraulically operating a clutch is provided with: a clutch master cylinder which generates a hydraulic pressure depending on the operation (i.e., the depression) of a clutch pedal which is defined as a clutch operating member; and a clutch operating cylinder which operates the clutch as a result of transmission of a hydraulic pressure from the clutch master cylinder. Further, Published Unexamined Japanese Patent Application No. 83518/1987 discloses an apparatus in which a flow limiting (or a flow restricting) device is interposed between the clutch master cylinder and the clutch operating cylinder. In this apparatus, in switching the clutch from a disengaged state to an engaged state, the flow amount of the operating fluid to be pushed out of the clutch operating cylinder is reduced at a point when the clutch attains a clutch-slipping state (or condition). It is thus so arranged that, even if the driver of the vehicle rapidly releases the depression of the clutch pedal, the clutch will not be suddenly engaged.
The above-described flow limiting device has a cylinder portion which comprises: a first fluid chamber on one axial side in communication with the clutch master cylinder; a second fluid chamber on the axially opposite side in communication with the clutch operating cylinder; and a piston for partitioning the first and the second fluid chambers. In that passage inside the flow limiting device which communicates from the first fluid chamber to the clutch master cylinder, there is interposed a throttle valve. The throttle valve has a valve body with a valve stem which projects through a guide hole formed in a peripheral wall of the cylinder portion so that the valve stem comes into contact with a peripheral surface of the piston. In this arrangement, the area of passage or flow is limited or restricted by the movement of the valve body in a direction in which the valve stem extends. When the piston has moved to a position in which the clutch attains the clutch-slipping state, the valve stem faces a recessed groove which is formed in the periphery of the piston. The valve body is then moved by the urging force of a spring in a direction in which the valve stem is inserted into the recessed groove, i.e., in the direction in which the valve stem extends. In this arrangement, in a position in which the clutch attains the clutch-slipping state, the area of the passage is throttled to thereby restrict the movement of the piston toward the first fluid chamber. The flow amount of the operating fluid to be pushed out of the clutch operating cylinder is thereby reduced.
In the above-described flow limiting device, there is employed a throttle valve which mechanically moves depending on the position of the piston. Therefore, it is likely to be influenced by the viscosity of the operating fluid due to the change in temperature. In other words, when the operating fluid is low in temperature and is therefore high in viscosity, even if the valve stem faces the recessed groove of the piston, the valve body will not move with a good response in the direction in which the valve stem extends. This is due to the fact that the valve body receives the viscous resistance of the operating fluid. As a result, the piston will sometimes move in a state in which the valve stem has not sufficiently been inserted into the recessed groove. In addition, the valve stem will sometimes be subjected to gouging or scratching inside the guide hole. In any of the above cases, sufficient throttling characteristics cannot be obtained in the clutch-slipping state. In addition, since the valve stem slides along the periphery of the piston while receiving an urging force of the valve spring, the top end of the valve stem wears away or is gouged (or scratched). Consequently, the throttling amount of the passage varies, and the wear may give rise to mechanical troubles.
In view of the above points, the present invention has an object of providing an apparatus for hydraulically operating a clutch which has the following feature, without using an interlocking member such as a throttle valve which mechanically moves depending on the position of the piston as is the case with the above-described conventional apparatus. The feature is that the returning of the clutch to the engaged state can be delayed in the clutch-slipping state, whereby the apparatus can be operated surely and the durability of the apparatus is also improved.
In order to attain the above and other objects, the present invention is an apparatus for hydraulically operating a clutch comprising: a clutch master cylinder for generating a hydraulic pressure by depressing a clutch operating member; a clutch operating cylinder for switching the clutch from an engaged state to a disengaged state by a hydraulic pressure transmitted from the clutch master cylinder; a flow limiting device disposed between the clutch master cylinder and the clutch operating cylinder such that, in switching the clutch from the disengaged state to the engaged state by releasing the depression of the clutch operating member, a flow amount of an operating fluid to be pushed out of the clutch operating cylinder is reduced when the clutch attains a clutch-slipping state, the flow limiting device having a cylinder portion which comprises: a first fluid chamber on one axial side in communication with the clutch master cylinder; a second fluid chamber on an axially opposite side in communication. with the clutch operating cylinder; and a piston for partitioning the first and second fluid chambers, characterized in: that the piston has on a circumference thereof: an annular groove in communication with the second fluid chamber through a passage inside the piston; and a land on the axially opposite side of the annular groove; that a first passage and a second passage are provided as passages to communicate the second fluid chamber and the clutch operating cylinder, the first passage opening into the cylinder portion in a position away in the axially opposite side relative to the land when the piston is in one stroke position at which the clutch is disengaged, the second passage opening into the cylinder portion at a position coinciding with a position of the annular groove when the piston is in said one stroke position, such that, in an axially intermediate stroke position of the piston at which the clutch attains the clutch-slipping state, an opening of the second passage is closed by the land; and that the first passage has interposed therein a one-way valve having an orifice, the one-way valve having an easy flow direction in a direction toward the clutch operating cylinder.
According to the present invention, at the time of switching the clutch from the disengaged state to the engaged state, the communication between the second fluid chamber and the second passage is shut off by the land of the piston when the clutch has attained the clutch-slipping state. The clutch operating cylinder is thus communicated with the second fluid chamber only through the first passage. Here, since the flow of the operating fluid in the first passage becomes a counterflow relative to the one-way valve, the operating fluid that is forced out of the clutch operating cylinder is returned to the second fluid chamber only through the orifice of the one-way valve. The flow amount of the operating fluid to be pushed out of the clutch operating cylinder is thus reduced. In this arrangement, the one-way valve is not an interlocking member which mechanically moves depending on the position of the piston. Therefore, a sufficient throttling characteristics can be obtained at the clutch-slipping state even if the viscosity of the operating fluid is high at a low temperature. In addition, the flow amount will not change with the lapse of time. Therefore, the apparatus can be operated surely with a smaller possibility of failure and with an improved durability. Still furthermore, since that land of the piston which closes the second passage slides along an inner surface of the cylinder which is free from projections and recessions, the surface of contact will not be subject to gouging or scratching operation. This also secures a construction which is free from mechanical troubles.